Polarization catalyst poisons

Water and polar solvents poisoned the early catalysts used. 

Despite the fact that polar entities are catalyst poisons, a variety of acyclic olefins containing a heteroatom functional group can undergo metathesis in the presence of a suitable catalyst, although at a high catalyst level. These include unsaturated esters, ethers, ketones, amines, nitriles, halogens, etc. [14]. In particular metathesis reactions - including ethenolysis - of unsaturated fatty esters and fatty oils are of interest, as they have perspectives for the oleochemical industry [15]. 

Catalyst deactivation and regeneration. The activity of a rhenium-based catalyst in the metathesis of unsaturated esters is unavoidably limited by the complexation of the ester group to the active site [8]. Moreover, there are many routes that lead to deactivation of the catalyst. Polar compounds such as H2O or free acids, alcohols and peroxides, which might be present as an impurity in the substrate(s), can act as catalyst poisons. Other possible routes for the deactivation of rhenium-based catalysts include (i) reduction of the rhenium below its optimum oxidation state (ii) adsorption of (polymeric) product molecules on the surface of the catalyst, blocking the active sites (iii) reductive elimination of the metallacyclobutane intermediate [59]. Even when the greatest care is taken, deactivation of the rhenium catalyst cannot be avoided. 

Solution processes often comprise an anionically catalysed polymerisation process. The catalyst is highly sensitive to the presence of polar impurities in the feedstream, particularly water. Therefore, it is essential that the solvent and monomers are free of such catalyst poisoning species. Even less sensitive catalysts, such as the Ziegler-Natta types, require a purification step on the feedstock prior to the reaction stage. 

Almost all polar substances exert a strong negative influence on the polymerization. COS and hydrogen sulfide, particularly, are considered to be strong catalyst poisons, of which traces of more than 0.2volppm affect a catalyst s activity. Neither the solvent nor the gaseous monomer should contain water, carbon dioxide, alcohols, or other polar substances in excess of 5 ppm. Purification may be carried out by means of molecular sieves. 

In all of the ethylene polymerization processes, the catalyst is sensitive to feed impurities and is poisoned by most polar compounds. Many of the properties of the polymer are determined by polymerization conditions, but catalyst composition and condition are critical determinants as well. 

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